Polymer-coated heat-sealable packaging material, method for manufacturing the same and a closed package made thereof

ABSTRACT

The invention relates to a manufacturing method of a heat-sealable packaging material, a polymer-coated packaging material manufactured using the method, and a sealed package to be manufactured of the material. The packaging material comprises a fibre base, such as a packaging board ( 5 ), and an inner, extruded water vapour barrier layer ( 2, 9 ) on both sides, containing high-density polyethylene (HDPE), and an outer heat sealing layer ( 1, 10 ), the material of which is, for example, low-density polyethylene (LDPE). Further, polymeric oxygen barrier layers ( 7 ) can be incorporated in the packaging material, the material of the oxygen barrier layers being, for example, ethyl vinyl alcohol copolymer (EVOH) or polyamide (PA). With the HDPE layer, the permeation of water vapour is prevented to both directions, and the location of the extruded layers on both sides of the fibre base reduces the curling of the material. The packages according to the invention comprise carton, box and bag packages for dry and liquid foods.

[0001] The invention relates to a method for manufacturing aheat-sealable packaging material, in which method both sides of a fibrebase are provided with a polymeric heat sealing layer. Further, theinvention relates to polymer-coated packaging material acquired by themethod and to a package manufactured of it by folding and heat sealing.

[0002] Tightness is required especially from packaging materials usedfor packaging foods for preventing the premature deterioration of theproduct. Typically, simultaneous oxygen and water vapour tightness isrequired from packages for liquid foods, such as milk products andjuices, whereas only good water vapour tightness is an essentialrequirement in packages for some dry foods. Unlike oxygen tightness,water vapour tightness can also be of importance not only for theprotection of the packed product, but also for the protection of thepackaging material itself, its fibre base and the polymeric coatinglayers.

[0003] Fibre-based packaging materials nowadays comprise differentmulti-layer coatings, in which different layers perform differentfunctions. In such coatings, polymers forming a good oxygen barrier areespecially ethylene vinyl alcohol copolymer (EVOH) and polyamide (PA).High-density polyethylene (HDPE) can be mentioned as a polymer renderinga good water vapour protection. In a multi-layer coating, low-densitypolyethylene (LDPE) has most often been used as the polymer for thetopmost heat sealing layer, which also prevents the permeation of watervapour, although not as well as HDPE. In addition, polymeric bindingagents may be needed in the multi-layer coatings of the packagingmaterial to bind the layers to each other and to the fibre base,depending on the polymers used.

[0004] In practice, the folding of polymer-coated packaging material tosealable packages by jointing has always been done so that themulti-layer coating with incorporated oxygen and water vapour barrierlayers becomes the interior surface of the package. The opposite side ofthe material, which will be the exterior surface of the package, isprovided at most with a heat sealing layer of LDPE or a correspondingpolymer, which gives protection against external moisture and makes itpossible to joint together the opposite edges of the packaging blankthat overlap each other in the folding. Especially in packages forliquid products, it has been the intention to prevent the packed productfrom wetting the fibre base and to leave the exterior surface of thepackage as free as possible for printing and patterning.

[0005] However, it has now been noted that problems may arise fromplacing the polymeric oxygen and/or water vapour barrier layers to theinterior surface of the package so that the protection of the exteriorsurface of the package is left for the heat sealing layer only.Especially in tropical countries, in which the temperature and relativehumidity of air are high, the ability of the most generally used LDPE toprevent the penetrating moisture is insufficient. The location of thebarrier layers in the present packaging materials also has the drawbackthat the material prevents the permeation of water vapour and moisturein different ways in different directions.

[0006] A second problem concerning the HDPE water vapour barrier formedby extrusion has been the curling of the coated board, which is causedby the post-crystallisation of the HDPE occurring after the extrusion.

[0007] It is the object of the present invention, which is based on theabove-mentioned observations, to solve the problem caused by the presentfibre-based packaging materials and the insufficient water vapourbarrier of packages joint from these. It is characteristic of the methodof the invention for manufacturing the packaging material that apolymeric water vapour barrier layer containing high-densitypolyethylene (HDPE) is extruded on both sides of the fibre base so thatthe water vapour barrier layer is left between the fibre base and theouter heat sealing layer in the packaging material.

[0008] Respectively, it is characteristic of the packaging material ofthe invention that it comprises the fibre base and the inner extrudedpolymer layer containing high-density polyethylene (HDPE) and acting aswater vapour barrier on both sides, and the outer heat sealing layer.

[0009] According to the invention, a water vapour barrier is arranged onboth sides of the packaging material so that the fibre base is protectedfrom both the moistening caused by the packed product and the moisturepenetrating from the outside of the package. The added water vapourbarrier also has the effect that moisture is not able to influence theoxygen barrier layers possibly incorporated in the material from eitherdirection, the said oxygen barrier layers being located so that they areleft between two water vapour layers.

[0010] Compared with the state of the art, the solution of the inventionintensifies the protection of the packaae against moisture penetratingfrom the outside so that, even in hot and damp circumstances, the baseboard and the possible interior barrier layers of the package areprotected against moisture, thus retaining the properties whichultimately ensure also the preservation of the product in the package.

[0011] The curling problem related with the packaging material is againreduced or even totally eliminated so that, in accordance with theinvention, the curling tendencies of the HDPE water vapour barrierlayers extruded on both sides of the fibre base cancel each other out.At best this is realised as the HDPE layers on the opposite sides of thefibre base are approximately of the same thickness.

[0012] As the packaging material manufactured according to the inventionis folded to a package, it may even make no difference for the watervapour barrier, which side of the material will be the exterior surfaceand which side the interior surface of the package. Thus, one embodimentof the invention is to provide a polymeric heat sealing layer on bothsides of the fibre base and a barrier layer preventing the permeation ofwater vapour beneath it so that the structure of the material issymmetrical. In this case, the material can come to the folded packageeither way, irrespective of the barrier properties or other demands madeon the package. In addition, a significant additional advantage is thatthe permeability of water vapour in the different directions of thepackage, i.e. from the outside into the package or from the inside outof the package, is the same.

[0013] The invention is further advantageously applicable so that atleast one polymeric layer forming the oxygen layer is further providedon the one side of the fibre base. Such a packaging material, which isespecially suitable for packages for liquid foods, is folded to apackage so that the oxygen barrier remains inside the fibre base of thepackage in a way, which is in itself conventional. The interiormulti-layer coating of the package then forms the oxygen and watervapour barrier protecting the product from the open air and the fibrebase from the packed product, at the same time as the added externalbarrier layer of the package protects the fibre base and the oxygenbarrier from moisture penetrating from the outside, thus ensuring thatthe package keeps its barrier and other properties during thepreservation time required from it.

[0014] The water vapour permeability of the high-density polyethylene(HDPE) used as the polymer preventing the permeation of water vapour inthe water vapour barrier layer is lower than that of conventional heatsealing polymers. The water vapour permeability of the coating can alsobe controlled with the help of layer thickness. HDPE combined with themost common heat sealing polymer LDPE stays imperforated in the heatsealing.

[0015] According to the invention, the weight of the HDPE layer formingthe water vapour barrier can be, for example, 5-50 g/m², preferably 7-30g/m², and most preferably 10-20 g/m². An efficient water vapour barrierpolymer is Borstar-HDPE, the density of which is approximately 0.963g/cm³. It is possible to blend pigments or, for example, ferrouscompounds, such as FeO to the water vapour barrier layer to captureoxygen.

[0016] A preferable polymer of the polyolefinic heat sealing layer islow-density polyethylene (LDPE), the layer weight of which can be, forexample, 5-50 g/m², preferably 5-30 g/m², and most preferably 7-20 g/m².If a second suitable polymer is blended with LDPE in the heat sealinglayer, such as polypropylene or polybutene, a jointing can be achieved,which opens when pulling off.

[0017] In the invention, LDPE refers to low-density polyethylene, thedensity of which is typically approximately 0.92 cm/³, and at most 0.94g/cm³. Polyethylenes with a higher density than this are high-densitypolyethylenes (HDPE) in connection of the invention.

[0018] The weight of the EVOH or PA oxygen barrier layer to be possiblyincorporated in the packaging material of the invention can be, forexample, 3-15 g/m², preferably 5-10 g/m².

[0019] The packaging material according to the invention can be formedof a packaging board (paperboard/cardboard), the weight of the fibrebase of which can vary between 130 and 500 g/m², being most preferablybetween 170 and 300 g/m². For example, a three-layer board generallyused, for example, in polymer-coated packaging boards, is suitable to beused as the fibre base; in the three-layer board, the thicker layer ofchemi-thermomechanical pulp (CTMP) is located between two thinner layersof sulphate pulp. In addition, the invention covers packaging papers, inwhich the weight of the fibre base is generally 20-120 g/m², mostpreferably 40-100 g/m².

[0020] It is characteristic of the sealed package of the invention,which is manufactured of a packaging material comprising a fibre baseand polymeric heat sealing layers on both sides by folding and heatsealing that, in the material, under the outer heat sealing layer onboth sides of the fibre base there is an extruded polymer layercontaining high-density polyethylene (HDPE) and acting as water vapourbarrier.

[0021] The package of the invention also advantageously comprises apolymeric oxygen barrier layer, which remains inside the fibre base ofthe packaging material, such as packaging board or paper. Thus, thepackage contains on its interior surface the oxygen barrier layer, theHDPE water vapour barrier layer, and further, the heat sealing layerforming the interior surface of the package. The exterior surface of thepackage includes the HDPE water vapour barrier layer and the heatsealing layer, and optionally also the exterior surface of the packagecan be provided with an oxygen barrier. Suitable oxygen barrier polymersare EVOH and PA, and suitable heat sealing polymer is LDPE.

[0022] The package of the invention can especially be a carton or boxpackage formed of polymer-coated board or a bag package formed ofpolymer-coated paper. Especially dry and liquid foods can be mentionedas products to be packed.

[0023] The invention is next described in more detail with the help ofexamples, referring to the enclosed drawings, in which

[0024]FIGS. 1-9 present the layer structures of packaging materials ofthe invention as nine different embodiment alternatives,

[0025]FIG. 10 presents a tin package of the invention, manufactured ofpackaging board by folding and heat sealing, and

[0026]FIG. 11 presents the results of curling measurements performed, inform of a bar graph.

[0027] The polymer-coated packaging board shown in FIG. 1 comprises inthe said order the LDPE heat sealing layer 1, the HDPE water vapourbarrier layer 2, the fibre base 5, which is, for example, a three-layerboard consisting of two sulphate pulp layers and of a CTMP layer betweenthem, the HDPE water vapour barrier layer 9, and the LDPE heat sealinglayer 10. The superimposed LDPE and HDPE layers 1, 2; 9, 10 on bothsides of the fibre base 5 are formed by coextrusion. The weight of thefibre base 5 is, for example, 250 g/m², the weight of both the HDPElayers 2, 9 is, for example, 15 g/m², and the weight of both the LDPElayers 1, 10 is, for example, 15 g/m². Thus, the board has a completelysymmetrical structure, in which case either one of the LDPE layers 1, 10can be the exterior surface of the package to be folded from the board,and either one can be the interior surface. The board is especiallysuitable for packaging dry foods in countries, in which the climate iswarm and damp.

[0028] The packaging board shown in FIG. 2 comprises in the said orderthe LDPE heat sealing layer 1, the HDPE water vapour barrier layers 2,the fibre base 5, the EVOH oxygen barrier layer 7, the binding agentlayer 8, for example, of graft polyethylene, the HDPE water vapourbarrier layer 9, and the LDPE heat sealing layer 10. The polymer layerson both sides of the fibre base 5 have been produced by coextrusion. Theweights of the LDPE, HDPE and fibre layers 1, 2, 5, 9 and 10 cancorrespond to the board in FIG. 1. The weight of the EVOH layer 7 canbe, for example, 5 μm², and the weight of the binding agent layer 8 canbe, for example, 5 g/m². The board is meant to be folded to a package sothat the LDPE layer 1 mentioned first will be the exterior surface ofthe package and the LDPE layer 10 mentioned last will be its interiorsurface, in which case the EVOH oxygen barrier layer 7 comes to theinterior of the fibre base 5 in the package. The board is especiallysuitable for packaging liquid foods so that the HDPE layers 2, 9 on bothsides of the fibre base give the necessary moisture protection to theother layers of the board at the same time as the EVOH layer 7 forms theoxygen barrier protecting the product.

[0029] The packaging board according to FIG. 3 differs from the oneshown in FIG. 2 only in that, instead of EVOH, the PA layer 6 isprovided as the oxygen barrier. The suitable weight of the PA layer 6is, for example, 7 g/m².

[0030] In FIG. 4, there is further shown an embodiment of the boardaccording to the invention, in which the oxygen barrier is formed ofsuperimposed PA and EVOH layers 6, 7. The weight of both these layers 6,7 can be, for example, 5 g/m². In the board according to FIG. 5, theoxygen layer 7′ is a blend of EVOH and PA.

[0031] In FIG. 6, there is shown a packaging board, which comprises inthe said order the LDPE heat sealing layer 1, the HDPE water vapourbarrier layer 2, the binding agent layer 3, the EVOH oxygen barrierlayer 4, the fibre base 5, which is a three-layer board similar to theembodiment in FIG. 1, the EVOH oxygen barrier layer 7, the binding agentlayer 8, the HDPE water vapour barrier layer 9, and the LDPE heatsealing layer 10. The polymer layers on both sides of the fibre base 5are most preferably produced by coextrusion. Respectively, the weightsof the layers are, for example, the following: the fibre base 250 g/m²,the LDPE layers 15 g/m, the HDPE layers 15 g/m², the EVOH layers 5 g/m²,and the binding agent layer 5 g/m². The board is thus of a symmetricalstructure, and when folding it to a package, its either side can be theexterior surface of the package and either side the interior surface.

[0032] The packaging materials shown in FIGS. 7-9 are variations of thematerial according to FIG. 1. Thus, in the heat sealing layer 1′, 10′ ofFIG. 7, LDPE has been replaced by a blend of it and polypropylene (PP).In the water vapour barrier layers 2′, 9′ of FIG. 8, a second component,for example, an oxygen capturing agent, such as ferro(II)oxide, or someother ferrite compound, has been blended with HDPE. In FIG. 9, insteadof the packaging board, there is shown a packaging paper, the weight ofthe fibre base 5′ of which is, for example, 80 g/cm³. Where thepolymeric coating layers are concerned, this packaging paper is similarto the board in FIG. 1.

[0033] In FIG. 10, there is shown an example of the package 11 of theinvention, which is formed of a packaging blank by folding and heatsealing, the blank being made of the packaging board according to FIG.2. The board is located in the package 11 so that on the exteriorsurface of the package outside the fibre base 5 there are thesuperimposed HEPD and LDPE layers 2, 1, in which case the four-layercoating 7, 8, 9, 10 containing the EVOH layer 7 remains inside the fibrebase 5. In the joints 12 of the package 11, the edges of the blank havebeen made to overlap, and the LDPE layers 1, 10 of the opposite surfacesof the board have been tightly heat sealed to each other.

[0034] The package 11 according to FIG. 10 is as well suited for thepackage for liquid foods, such as milk products and juices, as for dryfoods, such as flour, flakes, cereals, etc. According to the invention,essential in the package 11 is especially the protection of the packedproduct and the fibre base 5 from moisture penetrating from the outsidewith the help of the water vapour barrier layer 2 outside the fibrebase.

[0035] Tests carried out with the new packaging boards according to theinvention have been further explained in the following examples.

EXAMPLE 1

[0036] An extrusion-coated packaging board was manufactured, the layerstructure of which was: LDPE layer, weight 5 g/m², HDPE layer, weight 15g/m², base board, weight 190 g/m², HDPE layer, weight 15 g/m², and LDPElayer, weight 5 g/m². The water vapour permeation of this board wasmeasured as 6.4 μm²/d at the temperature of 38° in the relative humidityof 90%. The curling of the board was clearly small.

EXAMPLE 2

[0037] An extrusion-coated packaging board was manufactured, the layerstructure of which was: LDPE layer, weight 10 g/m², HDPE layer, weight10 g/m², base board, weight 255 g/m², HDPE layer, weight 20 g/m², andLDPE layer, weight 25 μm². The water vapour permeation of this board wasmeasured to be 4.6 g/m²/d at the temperature of 38° C. in the relativelyhumidity of 90%. The result is the average of the permeation valuesmeasured in different directions.

EXAMPLE 3 (Reference)

[0038] For reference, an extrusion-coated board was manufactured, thelayer structure of which was: LDPE layer, weight 20 g/m², base board,weight 255 μm², HDPE layer, weight 20 g/m², and LDPE layer, weight 25g/m². The water vapour permeation of the board was 5.5 g/m²/d, onaverage. The board was clearly more curled than the boards in Examples 1and 2.

[0039] Curling Measurements

[0040] The enclosed FIG. 11 contains results of curling measurements, inwhich test strips were cut from extrusion-coated boards, the measures ofwhich were 250×15 mm, and which were let freely hang in testcircumstances at the temperature of 23° C. and in the relativelyhumidity of 50%. The strips were cut so that the results indicate thecurling in the direction transverse to the machine. The graphs 1-5present the measurement results of a reference test series, in which theone side of the base board with the weight of 230 g/m² was coated withthe LDPE layer, the weight of which was 15, 25, 35, 45, or 55 g/m².Further, the graphs 6-8 are reference measurements of boards coated onlyfrom the one side, in which the inner layer was HDPE and the outer layerLDPE. The graph 6 concerns the structure: base board, weight 295 g/m²,HDPE layer, weight 35 g/m², and LDPE layer, weight 15 g/m². The graph 7concerns the structure: base board, weight 230 g/m², HDPE layer, weight40 g/m², and LDPE layer, weight 15 g/m². The graph 8 concerns thestructure: base board, weight 295 g/m², HDPE layer, weight 15 g/m², andLDPE layer, weight 5 g/m². When comparing the graphs 6-8 with the graphs1-5, the tendency of HDPE to curl the board can be noted.

[0041] The graphs 9-11 concern boards extrusion-coated on both sides sothat, besides LDPE layers, the coatings comprise an HDPE layer either onthe one side of the board (graph 9) or on both sides (graphs 10 and 11).The graphs 10 and 11 represent the structures according to theinvention. The completely symmetrical structure shown in graph 11, inwhich the weight of the base board was 190 g/m² and with inner HDPElayers with the weight of 15 g/m² on both sides of the base board andouter LDPE layers with the weight of 5 g/m², had the smallest curling inthe direction of the machine. The board in question was the boarddescribed in Example 1.

1. Method for manufacturing a heat sealable packaging material, in whichmethod both sides of a fibre base are provided with a polymeric heatsealing layer, characterized in that a polymeric water vapour barrierlayer (2, 9) containing high-density polyethylene (HDPE) is extruded onboth sides of the fibre base (5) so that the water vapour barrier layerremains between the fibre base and the outer heat sealing layer (1, 10)in the packaging material.
 2. Method according to claim 1, characterizedin that the inner water vapour barrier layer (2, 9) containing HDPE andthe outer heat sealing layer (1, 10) are brought onto the fibre base (5)in one step by coextrusion.
 3. Method according to claim 2,characterized in that superimposed water vapour barrier and heat sealinglayers (2, 1; 9, 10) are coextruded on both sides of the fibre base (5)so that the layer structure of the packaging material thus achieved issymmetrical.
 4. Method according to any one of the preceding claims,characterized in that the heat sealing layers (1, 10) containlow-density polyethylene (LDPE).
 5. Method according to claim 1,characterized in that at least one extruded polymer layer (4, 6, 7)forming an oxygen barrier is incorporated in the packaging material. 6.Polymer-coated heat sealable packaging material, characterized in thatit is manufactured using a method according to claim 1 and that itcomprises a fibre base (5) and an inner extruded polymer layer (2, 9)containing high-density polyethylene (HDPE) and acting as a water vapourbarrier layer and an outer heat sealing layer (1, 10) on both sides ofthe fibre base.
 7. Packaging material according to claim 6, characerizedin that the heat sealing layers (1, 10) contain low-density polyethylene(LDPE).
 8. Packaging material according to claim 7, characterized inthat the material consists of layers, the order of which is: LDPE heatsealing layer (1); HDPE water vapour barrier layer (2); fibre base (5);HDPE water vapour barrier layer (9); and LDPE heat sealing layer (10).9. Packaging material according to any one of the claims 6-8,characterized in that the weight of the water vapour barrier layers (2,9) on both sides of the fibre base (5) is the same, and that also theweight of the heat sealing layers (1, 10) on both sides of the fibrebase is the same so that the layer structure of the packaging materialis symmetrical.
 10. Packaging material according to claim 6 or 7,characterized in that the material further comprises at least onepolymer layer (4, 6, 7), which forms an oxygen barrier.
 11. Packagingmaterial according to claim 10, characterized in that the oxygen barrierlayer (4, 6, 7) contains ethyl vinyl alcohol copolymer (EVOH), polyamide(PA), or a mixture of these.
 12. Packaging material according to claim11, characterized in that the material consists of layers, the order ofwhich is: LDPE heat sealing layer (1); HDPE water vapour barrier layer(2); fibre base (5); one or a plurality of EVOH and/or PA oxygen barrierlayers (6, 7); polymeric binding agent layer (8); HDPE water vapourbarrier layer (9); and LDPE heat sealing layer (10).
 13. Packagingmaterial according to claim 6, characterized in that the water vapourbarrier layers (2, 9) are formed of Borstar-HDPE, the density of whichis approx. 0.963 g/cm³.
 14. Packaging material according to claim 6,characterized in that heat sealing layer (1, 10) is formed of LDPE, towhich polypropylene (PP) has been blended.
 15. Packaging materialaccording to claim 6, characterized in that the fibre base (5) is aboard, the weight of which is 130-500 g/m², preferably 170-300 g/m². 16.Packaging material according to claim 6, characterized in that the fibrebase (5) is a paper, the weight of which is 20-120 g/m², preferably40-100 g/m².
 17. Packaging material according to claim 6, characterizedin that the weight of the water vapour barrier layer (2, 9) is 5-50g/m², preferably 7-30 g/m², and most preferably 10-20 g/m². 18.Packaging material according to claim 6, characterized in that theweight of the heat sealing layer (1, 10) is 5-50 g/m², preferably 5-30g/m², and most preferably 7-20 g/m².
 19. Packaging material according toclaim 10, characterized in that the weight of the oxygen barrier layer(4, 6, 7) is 3-15 g/m², preferably 5-10 g/m².
 20. Sealed package, whichhas been manufactured by folding and heat sealing from a packagingmaterial manufactured using a method according to claim 1, characterizedin that, in the material, on both sides of a fibre base (5) under anouter heat sealing layer (1, 10) there is an extruded polymer layer (2,9) containing high-density polyethylene (HDPE) and acting as a watervapour barrier.
 21. Package according to claim 20, characterized inthat, inside the fibre base (5), between it and the water vapour barrierlayer (9) there is further at least one polymer layer (7), forming anoxygen barrier.
 22. Package according to claim 20 or 21, characterizedin that it is a carton package for liquid food, formed of polymer-coatedboard.
 23. Package according to claim 20 or 21, characterized in that itis a box package for dry food, formed of polymer-coated board. 24.Package according to claim 20 or 21, characterized in that it is a bagpackage formed of polymer-coated paper.